1. Field of the Invention
This invention relates to an apparatus for low concentration NOx (nitrogen oxides) combustion. More particularly it relates to a combustion apparatus such as boiler suitable for burning gas, oil and coal fed along the same axis and thereby reducing the amount of NOx contained in an exhaust gas.
2. Description of the Related Art
A burner for low concentration NOx combustion capable of burning oil and coal fed along the same axis is known, for example as disclosed in U.S. Pat. No. 4,545,307 or its corresponding Japanese patent application laid-open No. Sho 60-78207/1985, but a low NO.sub.x burner capable of burning three kinds of fuels, that is, gas, oil and coal along the same axis has not been known.
The reason is that in the case of such a burner for burning gas, oil and coal along the same axis, since a gas-feeding pipe is provided inside the whirling path of a tertiary air, its flow resistance is so high that it is impossible to afford an intense whirling to the tertiary air. Particularly in the case of a combustion system for low concentration NOx combustion, wherein denitration reaction is carried out within a flame, since such a tense whirling of a tertiary air is one of the requirements. Therefor, it has been impossible to achieve an adequate low concentration NOx combustion.
Another reason is that when coal is burnt, there is a danger of clogging due to ash adhesion or burnout of gas nozzle. Further, in the case of coal combustion boilers, since the depth of the furnace is usually large, a primary resonance frequency in the length direction of the furnace is so low that a combustion vibration is liable to occur.
As described above, a first problem of the prior art resides in that the whirling of the combustion air is hindered due to the gas-feeding pipe whereby it is impossible to afford the intense whirling and it is also impossible to achieve combustion for low concentration NOx making use of denitration reaction in the flame. A second problem of the prior art resides in that when coal alone is burnt, burnout of the gas nozzle or clogging of the gas nozzle hole due to coal ash occurs. Further, a third problem resides in that since coal is a difficultly combustible fuel, it is necessary to design the furnace so as to afford a large volume thereto; hence the depth of the furnace is necessarily so large that the resonance frequency of the primary mode lowers whereby the combustion vibration is liable to be induced at the time of gas combustion.